Carton based packages are commonly used within liquid food packaging. Typically, in order to form such packages, a web of carton based material is transported through a filling machine in which said web of carton based material is used to form closed containers, enclosing said liquid food.
One way of providing such packages is to feed the carton based web through a tube forming station, in which two longitudinal ends of the material web is sealed. The liquid product is introduced in the tube, whereby the individual packages are formed by subsequently sealing the upper and lower ends transversally. Flap folding may also be provided for achieving substantially planar upper and lower ends of the package.
Another way of forming carton based packages is to make use of a plastic top, e.g. being closed by a separate closing member, such as a cap. A carton based tube is connected to the plastic top, either simultaneously as the plastic top is molded, or as a separate step after manufacturing of the plastic top. The liquid food is then introduced into the tube/top construction, whereby the open end of the carton based tube is sealed and folded to form a closed package.
Sealing may be accomplished in various ways, however induction sealing has been proven to be a very efficient method. This principle is particularly beneficial for aseptic packages, of which the carton based material includes a thin layer of aluminum arranged in a sandwich construction between two layers of polymeric material for forming a solid barrier against the outer environment. When two ends of such carton based materials are arranged in close proximity to each other, as is the case when a transversal or longitudinal seal is to be achieved, a sealing bar may be pressed against the carton based material. The sealing bar includes a coil, and an electrical current is allowed to flow through the coil. The coil will consequently induce eddy currents, which causes heat to be generated in the aluminum foil. The generated heat will melt the polymeric materials adjacent to the aluminum foil, whereby these layers will bond to each other.
Examples of sealing bars are disclosed in for instance the Swedish Patent Publication SE 451 973 and the European Patent Application EP 0 642 914.
When manufacturing a sealing bar for this purpose, it is necessary to provide a planar surface for controlling close contact between the sealing bar and the packaging material. This is done by a multi-step process, involving a first step of providing the coil. The coil, normally made of copper with a rounded shape, is then grinded to form a smooth and planar surface. The coil is thereafter embedded in a plastic body, whereby the planar surface of the coil is aligned with a planar surface of the body. In this way, the sealing bar will have an entirely planar surface, exposing both the coil and the plastic body thus forming a support for the coil as well as a pressing surface for the sealing process.
However, it has been realized that the sealing bar, in particularly the planar surface, is among other things exposed to very high temperatures during sealing of packages. Such exposure will eventually cause the plastic body to deform from its original shape which may possibly result in a damaged interface between the coil and the plastic body.
Such defects may eventually also reduce the quality of the sealing obtained by the sealing bars, whereby the total quality of the liquid food packages is reduced.
Hence, it would be advantageous to provide a more robust sealing bar avoiding the above-mentioned drawbacks.